PHHP 96 - Tb or Not Tb? That Is the Question

Background: Infection prevention employs an algorithm that includes 3 negative sputum Acid Fast Bacilli (AFB) stains and 2 negative nucleic acid amplification (NAA) polymerase chain reaction (PCR) tests along with review of clinical findings and risk factors for Mycobacterium tuberculosis (MTb) to clear patients. NAA tests on smear negative specimens have a higher likelihood of false negative results. This has implications in the hospital setting and for public health.

Methods: Retrospective review of 648 unique MTb PCR data from January 1, 2020-November 31, 2022 with 16 positive results (8 had positive AFB smears; 16 grew MTb on culture). We identified one patient with 3 negative AFB smears and 3 negative MTb PCR tests who grew MTb on culture.

Results: The patient with discordant MTb PCR testing/culture results was a 16-year-old female who presented with 4 months of cough. She had recently immigrated to the US from Guatemala. She was febrile with tachycardia and tachypnea. Chest x-ray had patchy hazy opacities in the left mid to lower lung. She was placed on airborne precautions during specimen collection and precautions were removed after testing negative on AFB stain and PCR. The local health department took over management of the patient and she was treated with 4 drug anti-Tb therapy for 1 week and then it was discontinued based on the results. Approximately 2 weeks later, the patient’s AFB cultures became positive and it grew MTb. Patient was restarted on anti-Tb treatment.

Conclusions: MTb PCR testing has high sensitivity and high specificity to rule in or rule out MTb. However, a negative NAA test result does not exclude the possibility of a positive culture. Infection Preventionists must ensure that clinical findings and MTb risk factors are thoroughly reviewed along with the laboratory data before removing isolation precautions and be aware that false negative tests can occur.